Solution to Unusual Gamma Spec Spectrum

[Arsnews@aol.com]

Dear Radsafe:

I would like to thank all who have submitted theories/solutions to my request 
for help with the gamma spec spectrum.  I have received many postings and 
phone calls, and I am aware that there were a few discussion groups on this 
matter.  I certainly appreciate all the time and effort.  We now believe that 
it is a neutron interaction with Boron in the sample.  A description of the 
problem, Boron reaction, and final experiment follows.

I would also like to thank David Keefer - Duke Engineering and Services, 
Chris Hofmeyr - Council for Nuclear Safety, Lester Slaback - NIST, Ninni 
Jacob - Brown University, and Jean Charles Abbe - Laboratorie SUBATECH, for 
their responses as they were right on the mark.  A special thanks to Carl 
(Carlusa1@aol.com) for posting the spectrum on the web at 
http://sol.rutgers.edu/~carls/ for all to access.

Problem:
A water sample submitted to the laboratory for gamma spec analysis was found 
to have an unusual peak/peak shape at 470-485 keV.  Other naturally occurring 
peaks were present and of normal peak shape while this unusual peak was in 
the shape of a wide plateau (see http://sol.rutgers.edu/~carls/).  The sample 
was analyzed on an EG&G HPGe N type detector, 43.6% efficiency, resolution 
1.92 keV at 1.33 MeV, Co60.  Sample was a liquid with some susspended solids 
counted in a 2 liter marinelli for 150,000 seconds.  The sample was also 
analyzed on a similar but separate system with the same results.  A sample 
blank was analyzed on both systems with no indication of a peak in this 
region.

Theory:
The theory submitted is that this unusual peak is a result of a neutron-alpha 
reaction with stable B-10:  n + B-10 -> [B-11] -> Li-7 + a.  Several of the 
emails stated this would result in a Doppler broadened peak in the 470-485 
keV region with a flat top.

Verification:
Upon suggestion from Lester Slaback- NIST, a 2 liter solution of borax (20 
Mule Team) and DI water (~50 gm/l) was analyzed for 50,000 seconds.  The 
resulting spectrum contained a flat plateau/peak at 470-485 keV, identical to 
the peak identified in the original sample.  As ARS of New Mexico, located in 
Los Alamos (elevation ~7200 ft) does not have any neutron producing sources 
in house or in the immediate vicinity, it is believed that the source of the 
neutrons for this reaction is most likely cosmic/environmental.  The original 
sample has been analyzed at the ARS, Inc. laboratory located in Baton Rouge, 
LA (elevation ~100 ft).  The peak at 470-485 keV was present but at a 
significantly lower intensity.  A borax solution is to be counted soon for 
comparison.

Again, thanks to all that responded to this problem, and I apologize to 
anyone that posted that I was not able to respond to.

Steven Wish
American Radiation Services of New Mexico
www.amrad.com
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